The present invention relates to a multiplexer for an ATM network, e.g., such as the one described in European Patent Specification No. 0 618 749.
The ATM technique (Asynchronous Transfer Mode) makes it possible to transmit data of different types (sound, image, files etc.) on the same carrier. These data are arranged in packets called cells which have a standardized length of 53 bytes. An address contained in the header and formed by two fields, the VPI (Virtual Path Identifier) and the VCI (Virtual Channel Identifier) ensures the routing of the cells within the ATM network and thus permits of setting up a connection between a calling subscriber and a called subscriber. The payload of the transmission frame is called the ATM path. A succession of ATM cells which have the same VPI constitutes a virtual path VP. The route followed by a succession of ATM cells for the same VPI and the same VCI forms a virtual channel VC.
However, although the basic format of the cell is the same for all the data of different kinds, each of the connections one has to provide for each kind has its own characteristics in terms of traffic and Quality of Service (QoS). The characteristics, (including price . . . ) of each connection are to be specified in a detailed manner in the contract which binds the user of the ATM connection and the operator of the network. This contact binds both the user and the network provider:
The network provider agrees to transmit the user's data (ATM cells) with a certain service quality (specified in terms of average or maximum cell transmission time, number of error cells, number of lost cells etc. . . . ), for a traffic pattern specified in the contract and at a price fixed in the contract.
The user undertakes to pay for the data transmission service provided for him by the network, and is advised that if he does not comply with the traffic pattern specified in the contract, the network will no longer guarantee the service quality (for example, the network may destroy cells which are "outside" the traffic pattern fixed by the contract),
For accessing the ATM network, one is thus confronted with rate constraints and with problems of jitter both as regards the virtual path VP and as regards the virtual channel VC. The jitter which manifests itself by an irregularity in the time of appearance of the cells is particularly annoying for applications called real-time applications, for example, for video services.